I was reading guidlines for packing with dogs. I think it was on the Traildog group (
Traildog : A list for hiking & backpacking dogs)...can't remember specifically. They suggested that after training and conditioning, an adult dog can carry 20-25% of its body weigth. I think they suggested starting with small amounts (maybe 10%) and working up. So in an adult dog, start with 8 pounds (approximately 1 gallon water), and work up to a comfortable weight.
Obviously, I'm no expert, and each dog is different. This is just a random factoid that I recall.